Preparation of Group II metal alkyls

ABSTRACT

A method of preparing high purity dimethyl cadmium or dimethyl zinc suitable for use in the deposition of Group II-VI epitaxial layers, which consists of forming an adduct of the metal alkyl with a non-chelating tertiary amine containing at least two tertiary amino groups per amine molecule, and subsequently dissociating the adduct to liberate the metal alkyl as a vapor. The adducts formed during the preparative method are found to dissociate readily on heating and yet are substantially involatile and so do not contaminate the liberated metal alkyl. A preferred amine suitable for use in the preparative method is 4,4&#39; bipyridyl.

The present invention relates to the preparation of Group II metalalkyls, especially in a high purity form.

Epitaxial layers of metals either in elemental form or contained insemiconductor compounds are being increasingly used in the electronicsindustry. Elements such as cadmium, zinc, gallium and indium may forexample be deposited on substrate from their respective di- ortri-alkyls by thermal decomposition of the alkyls in the vapour phase togive the required epitaxial layers.

It is well-known that the presence of impurities in epitaxial layers canhave a profound effect on the electrical properties of such layers.Generally speaking it is therefore very desirable to use in theepitaxial deposition process metal alkyls of the highest purity(although controlled impurities such as those provided by p- or n- typedopants may be intentionally added).

A method for the purification of dimethylcadmium has been disclosed byG. B. Coates et al, J Chem Soc, 1962, 3340. In this method an adduct ofthe alkyl is formed with 2, 2'-bipyridyl. This adduct can be purified byrecrystallisation and dimethylcadmium can be liberated fairly rapidlyfrom it in vacuo at about 70° C. One disadantage of this method is thatthe dimethylcadmium so produce is sometimes contaminated withsignificant amounts of 2, 2'-bipyridyl. This is particularly the case ifhigher temperatures are used for recovering the dimethylcadmium. since2, 2'-bipyridyl contains the group V atom nitrogen, contamination of thedimethylcadmium in this way may have particularly serious consequencesfor semiconductors grown from this material since group V elements canact as p- type dopants.

The present invention seeks to overcome the above disadvantage byproviding in a first aspect, a method of preparing a metal alkyl ofgeeeral formula MR¹ R² in which M is selected from cadimum and zinc andR¹ and R² are independently selected from C₁ -C₈ alkyl, which comprisesthe steps of:

(a) forming an adduct of the metal with a Lewis base comprising atertiary amine aving at least two tertiary amino groups per moleculewhose nitrogen atoms are capable of co-ordinating with M, and

(b) dissociating the adduct to liberate the metal alkyl at a pressure Pand at a temperature below the sublimation temperature or boiling pointof the tertiary amine at pressure P as the case may be, wherein thetertiary amine has a molecular structure whose nitrogen atoms, forgeometric reasons, co-ordinate with different atoms M such that theadduct formed has a vapour pressure of less than 1.33 Pa at itsdissociation temperature at that pressure.

Preferably the vapour pressure of the adduct is less than 1.33 Pa at 20°C., most preferably at 3° C., above the dissociation temperature of theadduct at that pressure.

R₁ and R₂ are preferably independently selected from ethyl and methyl,and are most preferably methyl since dimethyl cadmium and dimethyl zincare generally considered to be the most useful alkyls of these metals insubsequent processes for the deposition of roup II-VI epitaxial layers.

The dissociation temperature of the adduct may be measured byconventional thermogram techniques at a pressure of 1.33 Pa (10⁻² torr),by increasing the temperature of the adduct at a steady rate (e.g. 10°C. per minute) and recording the temperature at which a sharp increasein the heat generated by the adduct begins to take place, signalling thebeginning of dissociation. The term "vapour pressure of the adduct"refers to the adduct itself and not its products of dissociation, andits volatility may be determined for the purpose of this invention byincreasing the temperature of the adduct to its dissociation temperatureat 1.33 Pa and observing whether it sublimes (if it is a sold at thattemperature) or boiis (if it is a liquid at that temperature). Ifneither boiling nor sublimation is observed, then the adduct will inaccordance with the method of the present invention have a vapourpressure of less than 1.33 Pa (10⁻² torr) at its dissociationtemperature at that pressure. Similarly, the procedure may be repeatedat 1.33 Pa at 20° C. or 30° C. above the dissociation temperature of thoadduct at that pressure.

The tertiary amine which is selected for use in the present method is anon-chelating amine, that is to say an amine whose molecules will notform a chelate structure with single MR¹ R² atoms. The diamine2,2'-bipyridyl is found to act as a Lewis base which forms a 1:1 moleadduct with diethylcadmium in which the nitrogen atoms of the diaminechelate the same cadmium atom as a bidentate ligand. The adduct which isformed by the present method does not have a chelate structure, that isto say no two nitrogen aroms within the amine molecule are capable ofchelating the same metal atom M as a bidentate ligand. This may beconfirmed by X-ray analysis of the crystalline structure of the adduct.The inventors have found that the effect of employing a non-chelatingrather than a chelating amine is that the adducts formed using theformer amines are substantially involatile at the typical temperaturesand pressure required for their dissociation. As a result, the metalalkyl product is less likely to be contaminated with the adduct or theamine, especially if the amine itself is of low volatility and the alkylis liberated as a gaseous product.

In terms of the structure of the non-chelating amine, this will usuallymean that either at least one of the nitrogen atoms forms part of ahererocyclic ring structure or adjacent nitrogen atoms in the aminemolecule are joined by a group which includes one or more aryl oralicyclic groups connecting between the nitrogen atoms. Preferably, theamine molecule will be free of any other atom capable of donatingelectrons to the atom M--on particular, the amine is preferably free ofany other group V or VI atoms within its molecular strucrure. For thisreason the amine is most preferably composed solely of the atomsnitrogen, carbon, and hydrogen.

Since the metal arom M will tend to form dative bonds with 2 nitrogenatoms and yet can only do so with one nitrogen atom on each tertiaryamine molecule, each M will tend to bond with nitrogen atoms fromseparate amine molecules. Thus, the adduct may tend to form with anoligomeric or polymeric structure which will be long-chain containingalternate amine and alkyl molecules in the case of a diamine andcross-linked in the case of an amine having three or more tertiary aminogroups per molecule. The main advantage of the possible formation of anoligomeric or polymeric adduct is that the adduct will be essentiallyinvolatile both below and at its dissociation temperature, so that itwill be safe to handle and will not contaminate the liberated alkyl.

The metal alkyl may be separated by simple distillation, e.g. by heatingthe adduct by an oil bath, and collecting the distilled alkyl in a coldtrap, or by fractionation, depending on the volatility of the amine.Preferably, separation is possible by simple distillation.

The required adduct specified above may be produced directly orindirectly, i.e. by direct reaction of the metal alkyl and the aminespecified or by the formation of another known (precursor) adduct, e.g.of the metal alkyl with a volatile solvent such as diethyl ether, andthen conversion into the required adduct by radical exchange uponaddition of the amine. In either case the impure metal alkyl or theprecursor adduct may be prepared by a known method. For example, theprecursor adduct dimethylcadmiumdiethyl etherate may be prepared by theknown method of reacting a cadmium dihalide, e.g. dichloride, with amethyl Grignard reagent, e.g. CH₃ MgI in diethyl ether.

The method of the present invention is preferably carried out belowatmospheric pressure, conveniently below 133 Pa (1 torr), so that thealkyl is released at low temperatures as a gaseous product and so iseasily separated from the adduct.

The tertiary amine is preferably a low volatility solid having a meltingpoint of between 50° and 200° C. and a boiling point of at least 200° C.at atmospheric pressure, and is preferably a solid at the dissociationtemperature of the adduct at 1.33 Pa. More preferably, the melting pointof the amine is at least 20° C., most preferably at least 30° C., abovethe dissociation temperature of the adduct at 1.33 Pa. In using suchpreferred amines, the risks of contaminating the metal alkyl productwith amine are reduced to a minimum where the alkyl is liberated as avapour.

Conveniently, the dissociation temperature of the adduct lies between20° C. and 120° C. at 1.33 Pa. The adduct will not therefore dissociateat roo temperature (15° C.). At temperatures above 120° C., there is anincreased danger that the metal alkyl will decompose explosively.

The tertiary amine which is used in the method of the present inventionmay fall into one of several classes of amine.

The amine may, for example, be of general formula I:

    X--Ar--X (I),

wherein each X is independently selected from tertiary amino substituentgroups and Ar is an arylene bridging group, provided that the aminosubstituent groups X are separated by at least two arylene ring atoms onthe Lewis ase molecule. Within the class, the amine is preferably ofgeneral formula II ##STR1## wherein X is as defined above, n is 0 or aninteger from 1 to 4, and each R³ is independently selected from C₁ -C₈alkyl. More preferably, X is NR⁴ R⁵ where R⁴ is C₁ -C₈ alkyl and R⁵ isC₁ -C₈ alkyl or ##STR2## provided that the number of nitrogen atoms inthe group X is from 1 to 3. Most preferably, R⁴ and R⁵ are independentlyselected from C₁ -C₈ alkyl and n 0s o. An example of this most preferredtype of amine is N,N,N',N'- tetramethyl-p-phenylene diamine.

Alternatively, the tertiary amino may comprise a pyridyl compoundsubstituted in the meta-ring position or, preferably, the para-ringposition by a tertiary amino group. The compound is preferablysubstituted in the para-ring position by X where X is --NR⁴ R⁵ asdefined above, and may be, for example, 4-dimethylamino-pyridine. Morepreferably, however, the pyridyl compound is of general formula III

    Py.sup.1 -(B).sub.m -Py.sup.2 III

wherein Py¹ and Py² are independently selected fromoptionally-substituted pyridyl provided that at least one of Py¹ and Py²is either optionally-substituted 3-pyridyl or optionally-substituted4-pyridyl, B is arylene, alkylene or cycloalkylene, and m is 0 or 1.Examples of suitable compounds of general formula III are 3,3'-bipyridyland, most preferably, 4,4'-bipyridyl . 4,4'-bipyridyl is especiallypreferred because when it forms an adduct with dimethyl cadmium thedissociation temperature of the adduct at 1.33 Pa is found to be some20° C. below the melting point of the amine.

As a yet further alternative, the tertiary amine may be anoptionally-substituted pyrazine, preferably one of general formula IV##STR3## wherein R³ and n are as defined above.

According to a second aspect of the present invention, there is providedan adduct which is the product of step (a) of the method according tothe first aspect.

It has been discovered that the non-chelating tertiary amines used inthe present method are especially suitable for use in obtaining alkylsof formula MR¹ R² in purified form. The adducts formed between thosemetal alkyls and these amines are, generally speaking, solids at roomtemperature (20° C.) which may, prior to separation of the metal alkylby distillation, be subjected to one or more of the techniques suitablefor the purification of solids, e.g. crystallisation from solution andzone refining.

It has been discovered that use of these non-chelating amines in theproduction of alkyls of formula MR¹ R² in high purity form isunexpectedly superior to the use of chelating analogues. It is beleivedthat this is because the non-chelating amines form an oligomer or apolymer complex structure from which the metal alkyl molecule may bemore easily dissociated upon decomposition. For example, as describedbelow, dimethyl cadmium may be separated more easily by simpledistillation from the adduct it forms with the non-chelating4,4'-bipyridyl than from the adduct it forms with the chelating 2,2'-bipyridyl.

According to a further aspect of the present invention, therefore, thereis provided an adduct of a metal alkyl of formula MR¹ R² with a Lewisbase comprising a tertiary amine, in which M is cadmium or zinc, R¹ andR² are independently selected from C₁ -C₈ alkyl, and the tertiary aminehas at least two tertiary amino groups per molecule whose nitrogen atomsare capable of co-ordinating with M, wherein the adduct contains atleast two molecules of the tertiary amine per molecule of the adduct.

Where the Lewis base is a tertiary diamine, the adduct will generally beof formula (MR¹ R²)_(x).L_(y) where L is the Lewis base, x and y areintegers, y is at least 2, and x is from (y-1) to (y+1). The adduct maybe in the form of a copolymer of the diamine and the metal alkyl whichhas an oligomeric or polymeric backbone containing at least two repeatunits of formula --MR¹ R² --.

The purified metal alkyl produced by the method of the present inventionmay be used in a known way particularly for the deposition on asubstrate of cadmium or zinc epitaxial layers either in single elementalform or as binary or multi-elemental II-VI compound layers byco-deposition with other suitable elements such as tellurium (e.g. asdiethyl telluride) and mercury (e.g. from mercury vapour) which togetherwith cadmium give cadmium mercury telluride. Essentially the cadmium orzinc is deposited from the cadmium or zinc alkyl by transporting thealkyl as a gas to a reaction vessel containing the substrate andthermally decomposing the alkyl in the vessel. Controlled impurities,e.g. n-or p-type dopants such as gallium or arsenic, may be co-depositedby adding trace amounts of the alkyls of these elements to the stream ofgas or gases to be thermally decomposed. Examples of methods of the useof cadmium alkyls to deposit cadmium epitaxially by thermaldecomposition are described for example in UK patent specification No.2078695A the subject matter of which specification is incorporatedherein by reference.

Examples of the method of the present invention and examples ofcomparative methods will now be described.

All solvents were carefully dried by distillation from sodiumdiphenylketyl and were degassed prior to use. Reactions were carried outin an atmosphere of "white spot" nitrogen purified by passed over acolumn consisting of Cr²⁺ on silica. All greaseless joints and taps wereemployed and manipulations were carried out using standard schlenk lineand catheter tubing techniques.

All Lewis bases were reagent grade and were purified by crystallisationfrom an ethereal solution such as diethyl ethor, or sublimation in vacuo(1.33 Pa).

Microanalyses (C,H, and N) were recorded on a Carlo-Erba 1106 carbon,hydrogen and nitrogen analyser. ¹¹³ Cd and ¹²⁵ Te n.m.r. spectra wererecorded on a Bruker Associates WM250 nuclear magnetic resonancespectrometer operating in the Fourier Transform mode with noise protondecoupling and ¹ H n.m.r. spectra on a Perkin Elmer R12B spectrometerand a Bruker Associates WP-80 nuclear magnetic resonance spectrometer.Mass spectra were recorded on the Associated Electrical Industries Ms902double focussing mass spectrometer. Trhermograms were measured on aPerkin Elmer DSC 2. Differential Scanning Calorimeter with a heatingrate of 10° C. min.⁻¹ Melting points were determined on anelectrothermal melting point apparatus in closed capillaries under argonand are uncorrected.

Example 1 (comparative) Preparation of impure dimethylcadmium Me₂ Cd

MeI (310 cm³) was slowly added to Mg (115 g) in diethyl ether (870 cm³)over 11 hrs. The Grignard reagent produced was added to CdCl₂ (215 g) indiethyl ether (502cm³) over 90 hrs at a temperature between 45° and 55°C.

The diethyl ether/Me₂ Cd mixture was distilled at 35°-85° C. over 6 hrs(1st fraction) and then at 100° C. over 4 hrs (2nd fraction) and at120°-123° C. for 4 hrs (3rd fraction). 2,2'-bipyridyl (125 g) was addedto the 1st fraction and then the 2nd and 3rd fractions were added.Diethyl ether was removed at room temperature to give yellow crystals(2,2'-bipyridyl-dimethylcadmium adduct) from which dimethylcadmium wasdistilled at 80°-95° C. under reduced pressure (1.33 Pa). The yield was123 g (75%). The dimethyl cadmium collected in a cold trap wascontaminated by the undissociated adduct which gave it a yellowcolouration, indicating that the adduct had a boiling/sublimation pointat 1.33 Pa of about 80° C. Differential scanning calorimetry andelectrothermal srudies shomed that the adduct began to dissociate at 67°C. (at 1.33 Pa) and melted at about 75° C.

EXAMPLE 2

N,N,N',N', -tetramethyl-p-phenylene diamine, TMPD (4.92 g) was added toMe₂ Cd produced as in Example 1 above in diethyl ether (25 cm³). Theother was evaporated from the resulting dark brown solution and benzenewas added to the residue until it dissolved. A few drops of petroleumether were added until crystals re-appeared. The petroleum ether andbenzene were removed by filtration. Further dissolution of the crystalsin petroleum ether followed by cooling to about 5° C. yielded highpurity crystals of the adduct of Me₂ Cd with TMPD. Nuclear magneticresonance ¹ H (NMR) and microanalysis studies revealed that the adductcontained Me₂ Cd and TMPD in the molar ratio 2:3 indicating the linkingof two or more molecules of TMPD with molecules of Me₂ Cd in a lineararrangement such as TMPD-Me₂ Cd-TMPD-Me₂ Cd-TMPD.

The adduct of Me₂ Cd and TMPD was then heated at about 80 ° C. in vacuo(10³¹ 2 torr, 1.33 Pa) to yield high purity dimethyl cadmium which wascollected in a cold trap. Differential scanning calorimetry andelectrothermal studies showed that the adduct melted at about 35° C. andbegan to dissociate (at 1.33 Pa) at about 33° C.

EXAMPLE 3

Impure dimethyl cadmium, Me₂ Cd, was produced as in Example 1 above.4,4'-bipyridyl (bipy) (2.35 g) was then added to the Me₂ Cd (2.11 g) intetrahydrofuran. The solution was then cooled to about 5° C. Crsytalswere removed from the solution and washed in petroleum ether. ¹ H nmrand microanalysis revealed that the adduct contained Me₂ Cd and bipy inthe molar ratio 1:1 indicating the linking of two or more molecules ofbipy by the same number of molecules of Me₂ Cd either in a lineararrangement such as by-Me₂ Cd-biPy-Me₂ Cd or a cyclic arrangement suchas ##STR4## The adduct of Me₂ Cd and bipy was then heated at about 110°C. in vacuo (10⁻² torr, 1.33 Pa) to yield high purity dimethyl cadmiumwhich was collected in acold trap. Differential scanning calorimetry andelectrothermal studies showed that the adduct melted at about 138° C. anbegan to dissociate (at 1.33 Pa) at about 83° C.

EXAMPLE 4

Dimethylcadmium (9 g, 6.32×10⁻² gmoles) was added in vacuo to a solutionof 4-dimethylaminopyridine (6.14 g, 5.03×10⁻² gmoles) in tetrahydrofuran(50 cm³). The frozen contents were warmed to room temperature todissolve any crystalline solid and then the adduct crystals were broughtout of solution by cooling to -30° C. overnight. The solvent and excessdimethylcadmium were filtered off under nitrogen, first of all at 0° C.and then at room temperature, until the crystals were dry. This produceda very pale brown crystalline solid.

¹ H Nuclear magnetic resonance and microanalysis studies revealed thatthe adduct contained Me₂ Cd and dimemethylaminopyridine in the molarratio 1:1.

The adduct of Me₂ Cd and 4-dimethylaminopyridine was then heated to 65°C. at 1.33 Pa, 5° C. above its dissociation temPerature at thatpressure, to yield pure dimethylcadmium, below the melting point of theadduct, which was collected in a cold trap maintained at a liquidnitrogen temperature. sublimation and melting was found to occur at andabove 70° C. (Yield of dimethylcadmium=57%).

EXAMPLE 5

Dimethyl Cadmium (6.27 g, 4.40×10⁻² gmoles) was added in vacuo to asolution of 3,3'-bipyridyl (5 g, 3.20×10⁻² gmoles) in diethyl ether (60cm³). The frozen contents of the reaction mixture were then warmed up toroom temperature to dissolve the crystalline solid present. The adductcrystals were brought out of solution by leaving at -30 ° C. overnight.The diethyl ether and excess dimethylcadmium were filtered off undernitrogen, first of all ° at 0° C. and then at room temperature, untildry, producing a pale brown crystalline solid.

¹ H Nuclear magnetic resonance revealed that the adduct contained Me₂ Cdand 3,3'-bipyridyl in the molar ratio 1:1.

The adduct of Me₂ Cd and 3,3'-bipyridyl was then heated to 75° C. invacuo (1.33 Pa) to yield pure dimethylcadmium which was collected in acold trap maintained at liquid nitrogen temperature. No sublimation ofadduct was found to occur up to 80° C. Me₂ Cd yield: 40%.

Dimethylzinc used in the following Examples 6-8 was prepared by thereaction of Grignard intermediate, methylmagnesium iodide and zincchloride in a high boiling point ether (e.g. isoamylether or isopentyIether) as outiined below:

(a) Preparation of MeMgI in Isoamylether

Iodomethane (78 cm³, 1.25 gmole) was added dropwise over a period of 4hours under the atmosphere of dry nitrogen to a suspension of magnesiumturnings (32 g,1.32 g atoms) in isoamyl ether 500cm³) containing tracesof 1,2 dibromoethane (2 cm³) as an initiator. The reaction mixture washeated under reflux on a water bath maintained ° at 45° C. for 6 hoursafter the complete addition of iodomethane. The reaction mixture wasthen evacuated at room temperature in order to remove unreactediodomethane (if present).

(b) Preparation of Dimethylzinc

MeMgI solution in isoamylether prepared as in (a) was added dropwise tothe suspension of dry zinc chloride (71.44 g) 0.52 gmole) in isoamylether (50cm³), under an atmosphere of dry nitrogen, over a period of 5hours in a three-necked-flask equipped with a mechanical stirrer, avigreux column, and a reflux condenser. After the complete addition ofMeMgI, the reaction mixture was refluxed over a water bath, maintainedat 60° C. for 8 hours.

Dimethylzinc was obtained by the distillation of reaction mixture atatmospheric pressure on a wax bath maintained at 180° C. (° 42.512° g,Yield=85%, B.pt of collected fraction 45°-46° C.)

EXAMPLE 6

Dimethylzinc (2.73 g, 2.862×10⁻² gmole) was distilled in vacuo into asolution of 4,4'-bipyridyl (2.41 g, 1.537×10⁻² gmole) in diethylether(100 cm³). A pale yellow microcrystalline precipitate was obtained whenthe contents of the reaction mixture were warmed to room temperature.The ether and excess dimethylzinc were removed by filtration under anitrogen atmosphere and by successive washings with diethyl ether cooledto 4° C. The adduct obtained was dissolved in dry tetrahydrofuran (40cm³) and the resulting solution was concentrated to half its volume byremoval of tetrahydrofuran in vacuo. Pure crystals of adduct wereobtained by cooling the concentrated solution to -30° C. overnight.

¹ H Nuclear magnetic resonance and microanalysis studies revealed thatthe adduct contained Me₂ Zn and 4,4'-bipyridyl in the molar ratio 1:1.

The adduct of Me² Zn and 4,4'-bipyridyl was then heated in vacuo (1.33Pa) to 90° C. to yield pure dimethylzinc which was collected in vacuo ina cold trap. The dissociation of adduct was found to occur between85°-90° C. No sublimation of adduct was found to occur upto 90° C.(Yield of dimethylzinc=68%).

Example 7

Dimethylzinc (2.02 g, 2.12×10² gmoles) was added in vacuo to a solutionof 3,3'-bipyridyl (3.29 g, 2.10×10⁻² gmoles) in diethyl ether (20 cm³).The frozen contents of the reaction mixture were then warmed to roomtemperature to obtain a pale yellow microcrystalline precipitate. Theether and excess dimethylzinc were removed by filtration under anitrogen atmosphere and by successive washings with diethyl ether cooledto 4° C. The adduct obtained was dissolved in dry tetrahydrofuran (50cm³) and resulting solution was concentrated to half its volume byremoval of tetrahydrofuran in vacuo. Pure crystals of adduct wereobtained by cooling the concentrated solution to -30° C. overnight.

¹ H Nuclear magnetic resonance and microanalysis studies revealed thatthe adduct contained Me₂ Zn and 3,3'-bipyridyl in the molar ratio 1:1.

The adduct of Me₂ Zn and 3,3'-bipyridyl was then heated to 90° C. invacuo (1.33 Pa) to yield pure dimethylzinc which was collected in a coldtrap maintained at a liquid nitrogen temperature. No sublimation ofadduct was found to occur up to 90° C.. (yield of dimethylzinc=98%).

EXAMPLE 8

Dimethylzinc (3.61 g, 3.77×10² gmoles) was added in vacuo to a solutionof 4-dimethylaminopyridine(3.24 g, 2.65×10⁻² gmoles) in diethylether(200 cm³). The frozen contents of the reaction mixture were then warmedto obtain a clear colourless solution which was stirred at roomtemperature for 30 minutes and then was subjected to concentration bythe removal of diethyl ether and excess dimethylzinc in vacuo. Purecrystals were obtained by cooling the concentrated solution (volume=50cm³) to -30° C. overnight.

¹ H Nuclear magnetic resonance and microanalysis studies revealed thatthe adduct contained Me₂ Zn and 4-dimethylaminopyridine in the molarratio 1:1.

The adduct of Me₂ Zn and 4 -dimethylaminopyridine was then heated to 80°C. in vacuo (1.33 Pa) to yield pure dimethylzinc which was collected ina cold trap maintained at a liquid nitrogen temperature. Thedissociation of adduct was found to occur between 75°-80° C. whereas thesublimation of adduct was observed at temperatures between 100°-110° C.

COMPARATIVE EXAMPLE 9

1,4 Dioxan (2.36 cm³) was added to dimethyl cadmium (3.7 g) in diethylether. The ether was removed by evacuation to leave a white crystallinesolid (4.3 g).

¹ H nmr studies revealed a 1:1 adduct of 1,4 dioxan and dimethylcadmium. The yield was 75%.

The adduct melted at 57° C. he adduct sublimed but did not give off Me₂Cd on heating at reduced pressure. The adduct is known to be a chelatestructure.

COMPARATIVE EXAMPLE 10

Ethylene diamine (1.1 cm³) was added to dimethyl cadmium (2.92 g) indiethyl ether (15 cm⁵).

The mixture was stirred at room temperature. A reaction occurred causingthe emission of gas resulting in the solution turning green/brown. Theether was removed by evacuation and a green/brown slurry was recovered.This was left overnight in a fume cupboard. It emitted more gasresulting in a dark green dry solid. Mass spectral analysis of the greensolid showed no sign of Me₂ Cd or indeed of any cadmium at all.

The conclusion reached was that the protons on the ethylene diamine aretoo acidic and result in the breakdown of Me₂ Cd.

COMPARATIVE EXAMPLE 11

Tetramethyl ethylene diamine (TMED) was added to Me₂ Cd (2.85 g) indiethyl ether 10 cm³). The ether was pumped off to reveal a yellowcrystalline solid. On heating under reduced pressure the crystalssublimed to produce a white solid at 70° C. ¹ H nmr studies andmicroanalysis showed the compound to contain Me₂ Cd and TMED in a 1:1rtio. Microanalysis confirmed the result. Differential ScanningCalorimetry revealed the adduct to have a melting point of 62° C. and asublimation temperature at 1.33 Pa of 70° C.

Dimethyl cadmium could not be dissociated from the adduct because of thechelate structure formed.

COMPARATIVE EXAMPLE 12

Triphenylamine (TPA) (6.54 g) was added to Me₂ Cd (1.86 g) in petroleumether (30 cm³). The resulting slurry was cooled to about 5° C. Nocrystals were isolated. The petroleum ether was removed by pumping andtoluene was added. The TPA dissolved in the toluene when placed in arefrigerator yielded white crystals. ¹ H nmr analysis showed them to beTPA (no Me₂ Cd). A study of shifts in tetrahydrofuran showed no sign ofcoordination between the TPA and Me₂ Cd.

COMPARATIVE EXAMPLE 13

Dimethylanaline (DMA) (9 cm³) was added to Me₂ Cd (5.5 g) in diethylether. The mixture produced no heat and no precipitation was observed.The solution was placed in a refirgerator. No crystals were produced.

The chemical shifts of the components were observed by ¹ H nmr to seewhether any changes had taken place. None were indicated.

We claim:
 1. A method of preparing a metal alkyl of general formula MR¹R², in which M :s selected from cadmium and zinc and R¹ and R² areindependently selected from C₁ -C₈ alkyl. which comprises the stepsof(a) forming an adduct of the metal alkyl with a Lewis base comprisinga tertiary amine having at least two tertiary amino groups per moleculewhose nitrogen atoms are capable of co-ordinating with M, and (b)dissociating the adduct to liberate the metal alkyl at a pressure P andat a temperature below the sublimation temperature or boiling point ofthe tertiary amine at pressure P as the case may be, characterised inthat the tertiary amine has a molecular structure whose nitrogen atoms,for geometric reasons, co-ordinate with different atoms M such that theadduct formed has a vapour pressure of less than 1.33 Pa at itsdissociation temperature at that pressure.
 2. A method according toclaim 1 characterised in that the vapour pressure of the adduct is lessthan 1.33 Pa at 20° C. above the dissociation temperature of the adductat that pressure.
 3. A method according to claim 2 characterised in thatthe vapour pressure of the adduct is less than 1.3 Pa at 30° C. abovethe dissociation temperature of the adduct at that pressure.
 4. A methodaccording to claim 1 characterised in that the Lewis base has a meltingpoint in the range 5° C. to 200° C. and a boiling point at atmospherepressure of more than 200° C.
 5. A method according to claim 1characterised in that the Lewis base is a solid at the dissociationtemperature of the adduct at 1.33 Pa.
 6. A method according to claim 5characterised in that the Lewis base is a solid at least 20° C. abovethe dissociation temperature of the adduct at 1.33 Pa.
 7. A methodaccording to claim 1, characterized in that the Lewis base comprises atertiary amine of general formula I

    X--Ar--X                                                   I

wherein each X is independently selected from tertiary amino substituentgroups and Ar is an arylene bridging group, provided that the aminosubstituent groups X are separated by at least two arylene ring atoms onthe Lewis base molecule.
 8. A method according to claim 7 characterisedin that the Lewis base comprises a tertiary amine of general formula II##STR5## wherein each X is as defined in claim 7, n is 0 or an integerfrom 1 to 4, and each R³ is independently selected from C₁ -C₈ alkyl. 9.A method according to claim 8 characterised in that each X isindependently selected from --NR⁴ R⁵ where R⁴ and R⁵ are independentlyselected from C₁ -C₈ alkyl, and n is
 0. 10. A method according to claim1 characterised in that the Lewis base comprises a pyridyl compoundsubstituted in the para- or meta- ring positions by a tertiary aminogroup.
 11. A method according to claim 10 characterised in that theLewis base is of general formula III

    Py.sup.1 --(B).sub.m --Py.sup.2                            III

wherein Py¹ and Py² are independently selected fromoptionally-substituted pyridyl provided that at least one of Py¹ and Py²is either optionally-substituted 3-pyridyl or optionally-substituted4-pyridyl, B is arylene, alkylene, or cycloalkylene, and m is 0 or 1.12. A method according to claim 11 characterised in that m is 0 and theLewis base is either 3,3'-bipyridyl or 4,4'-bipyridyl.
 13. A methodaccording to claim 10 characterized in that the pyridyl compound issubstituted in the para- ring position by a tertiary amino group X,which is independently selected from --NR⁴ R⁵ where R⁴ is C₁ -C₈ and R⁵is ##STR6## provided that the number of nitrogen atoms in each group Xis from 1 to 3, n is 0 or an integer from 1 to 4, and each R³ isindependently selected from C₁ -C₈ alkyl.
 14. A method according toclaim 1 characterised in that the Lewis base is anoptionally-substituted pyrazine.
 15. A method according to claim 14characterized in that the Lewis base is of general formula IV ##STR7##wherein n is 0 or an integer from 1 to 4 and each R³ is independentlyselected from C₁ -C₈ alkyl.